(257b) Determination of Hydration Lubrication Regime in Sparse Zwitterionic Monolayers
We explore the lubrication of sparse 2-methacryloyloxyethyl phosphorylcholine (MPC) monolayers in water, a monomer analog to experimentally studied zwitterionic polymer brush systems. Our results show two distinct shear regimes. The first is dominated by hydration lubrication, the mechanism suggested to underlie the extremely efficient tribological properties of e.g. nanoconfined hydrated ions, surfaces functionalized with polyzwitterionic polymer brushes, and mammalian synovial joints. Here, the presence of mixed water-monomer domains allows for high compressibility of such materials. Simultaneously, the ability of water to rapidly exchange between these mixed domains and bulk-like water layers facilitates a fluid-like response to shear. This fluid response is maintained while compressing systems by up to 50% the length of a single monomer yielding ultra-low friction coefficients. Upon compressing the system beyond this distance, a transition is observed to a second regime that is dominated by chain-chain interactions and more closely resembles the shear response of dense self-assembled monolayers. This high shear regime is characterized by low mobility of water molecules, interactions between chains from opposing monolayers, and an increased coefficient of friction. Systems operating in the first shear regime may represent the nano-environment within the experimentally studied brush systems that leverage the hydration lubrication mechanism to yield appealing tribological properties.
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